This invention relates generally to high bandwidth radio frequency transmitters, and, more specifically, to ultra wideband radio frequency upconverters wherein analog component nonlinearities may be pre-compensated to improve received signal quality.
As data-rate requirements necessary to support the wireless transmission of high-quality intelligence, surveillance and reconnaissance (ISR) information increase, so do the need for high bandwidth communications systems to support the downlink of this ISR information. One method proposed to enable these high data-rates is to increase wireless signal bandwidths to multi-gigahertz (GHz). Such a signal bandwidth can theoretically support much higher data-rates than traditional kilohertz (kHz) to megahertz (MHz) wide signals, while maintaining realistic modulation schemes that require only moderate signal-to-noise ratios (SNR).
However, existing commercial-off-the-shelf (COTS) microwave frequency components are not designed with the intention of supporting multiple GHz of instantaneous signal bandwidth. Even though these components may support the operation of signals with wide bandwidths, their magnitude and phase response across this band is non-linear. While a narrow-band signal may approach linear operation through an analog component, the wideband nature of a multi-GHz waveform means that some signal components may experience frequency-selective fading that differs significantly from other frequency components across the same instantaneous signal bandwidth. This results in distortion of the waveform and provides a challenge for data recovery techniques at the receiver.
Existing receiver signal equalization techniques may provide some amount of benefit compensate for these non-linearities, but are limited in their capabilities. First, since a receiver does not have information about the ideal transmitted data symbols, the receiver equalizer must blindly attempt to recover the ideal symbols of a signal constellation, then iteratively minimize the identified error. If the received signal is too degraded to determine these ideal symbols, the receiver equalizer will be unable to improve the signal and the received symbols will not be properly decoded. Secondly, as the amount of signal distortion increases due to the effects of non-linear responses in the transmitter and receiver analog hardware, in addition to distortion due to the power amplifier (PA) and wireless channel effects, the burden on the receiver equalizer may become more than it can reasonably compensate for. These facts drive the need for a type of transmitter equalization to improve the response of the signal due to analog component non-linearities.
Pre-distortion of a transmitted radio frequency waveform is necessary when compression of a waveform within a high power amplifier (PA) causes non-linear distortions in the passband signal. Because some amount of this distortion is dependent on the input signal power, temperature stability and PA specifications, given enough information a portion of the distortion can be deterministically modeled. Digital pre-distortion is the practice of purposefully distorting the otherwise ideal digital data before analog transmission to compensate for the deterministic compression caused by the PA.
An optimal solution to the degradation of signal quality of ultra wideband signals through non-linear analog components should be one which is low cost, straightforward to implement and does not require significant changes to existing communication systems. The prior art has been able to isolate some of these parameters independently, but has failed to optimize for all parameters at once.
In U.S. Pat. No. 3,755,754, a pre-distortion technique is described for the compensation of perceived distortion occurring in a signal due to PA non-linearities. In this case, the original signal is separated into two signals where one is distorted through the PA and the other amplified linearly. The signals are then combined with a phase inversion, causing a cancellation of some distortion components. However, this technique targets distortion caused only by the PA while requiring additional analog hardware otherwise unnecessary in the original transmitter.
In U.S. Pat. No. 6,288,610, a method and apparatus for signal pre-distortion is identified which implements a means to detect and compensate for distortion caused by a PA. This technique uses digital pre-distortion techniques, which minimizes the footprint necessary for overall pre-distortion. However, the method requires a second upconverter, PA and power combiner to realize the distortion compensation, significantly impacting the size and cost of the overall technique.
In United States Patent Application Publication 20050088230, an amplifier pre-distortion processing technique is described which uses look-up tables to implement a pre-distortion filter on both the in-phase and quadrature-phase components of a quadrature amplitude modulated waveform. Using look-up tables results in a pre-distortion technique that is less costly to implement and requires a smaller digital footprint than other approaches in the prior art. However, this implementation is used for pre-distortion of a transmitter power amplifier, not the non-linearities of analog components which result in signal degradation of wideband waveforms.
In United States Patent Application Publication 20070153884, a method for calculating the pre-distortion filter coefficients for a closed-loop feedback system is provided. This method updates the pre-distortion coefficients as the characteristics of a PA change over time. While this method is effective for real-time updating, it requires significantly more analog and digital hardware to support the additional downconversion and decoding of the PA-affected signal before transmission.
In U.S. Pat. No. 8,351,877, a digital pre-distortion system and method enables the pre-distortion of a wideband waveform for the distortion effects of a PA. While this invention does address the wideband applications of pre-distortion, the technique is only applied to compensate for PA effects, which results in an incomplete solution to distortion problems.